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Just a bit of context, I am a technology enthusiast. I love everything from software to hardware, Windows to Linux, Computer to Phone, and everything in between. So, I wouldn't be surprised is my electrical usage exceeds 50% of the entire household (8 people). To power all of the equipment I use, I use a pretty expensive surge protector, with 12 outlets on it. Recently, I have been very plain and simply, running out of outlets.

I have an extension cord with 3 outlets on it. Maybe, I thought, I could plug some of the things on my surge protector into the extension cord, and then the extension cord into the surge protector. I know, it's pretty dumb of me, but I went and did it, without checking to see if it was safe first. But I did, and everything seems to running fine. It's been running for about a 30 minutes now, and neither the surge protector or the extension cord are overheating.

However, after the fact, I am reading on websites about weather or not this is safe, and they're all saying it's not, even though everything is running perfectly fine. Thoughts? Something I'm not understanding?

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  • \$\begingroup\$ The danger (if there is any) is that something will overheat and fail or start a fire. Since you are a technology enthusiast, you simply MUST have a thermal imaging camera or thermal imaging adapter for your mobile phone. Use this to make sure none of your cables or power strips are getting too warm. If they aren't, then most likely, all is well. If anything is getting hot, then take steps to reduce the load on it, or at least investigate further. It may take things a long time to reach their final temperature. \$\endgroup\$ – mkeith Feb 22 at 3:56
  • \$\begingroup\$ Sorry to disagree with the closing reason. As I see this question, this is not an "how to use an extension cord" question. It is a (beginner) question on how to safely organize a "temporary" electrical system setup beyond what manufacturer instructions (aimed at "dumb consumers") specify. It is like asking if a given component can be used beyond its specs in the datasheet (but here the "datasheet", i.e. a document for professionals, is not provided). Voting to reopen. \$\endgroup\$ – Lorenzo Donati supports Monica Feb 23 at 9:42
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To be honest, I do it all the time. However, there are some things to keep in mind:

  • Never exceed the maximum power, current and voltage.
    • Voltage will normally not be a problem since this is (more or less) stable in a country. When you go abroad, check if they can be used.
    • current is most important, sum up all devices you connect on that extension cord AND all devices on extension connected to the the first extension cord. Take some margin, especially motors sometimes can use a lot of 'starting' current.
  • Don't forget when you switch on an extension cord (either by plugging it in, or by using a switch if there is on), ALL devices will be powered on at the same time, using possibly some additional extra current.
  • When you go towards the maximum current supported by the extension cord or in a warm environment or radiation (e.g. from the sun), make sure the cable is fully unrolled (meaning not bundled), since warmth cannot spread (updated according comment of FerryBig).
  • Apart from heating cables or connectors, a primary safety concern with an extension plugged into an extension of any type is that overall resistivity can become so large that even a full short at the furthest end of the cable may not trip a breaker. Another potential problem is that if the outlet is equipped with an earth-leakage circuit breaker safety device, that may also trip "on it's own" as the current in neutral wire is diminished due to excessive resistivity in the circuit. (addition from Kurja).
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  • \$\begingroup\$ @Kurja (I copied your edit to my answer as a comment): Apart from heating cables or connectors, a primary safety concern with an extension plugged into an extension of any type is that overall resistivity can become so large that even a full short at the furthest end of the cable may not trip a breaker. Another potential problem is that if the outlet is equipped with an earth-leakage circuit breaker safety device, that may also trip "on it's own" as the current in neutral wire is diminished due to excessive resistivity in the circuit. \$\endgroup\$ – Michel Keijzers Feb 22 at 10:29
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    \$\begingroup\$ Also consider external factors (like the heat from the sun) when calculating the maximum load, I once had a thermal fuse trip inside a rolled up cable when it was using less than 1A (at 220V) \$\endgroup\$ – Ferrybig Feb 22 at 15:35
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    \$\begingroup\$ Could you please edit the comment about the resistivity into your answer? This aspect is often unknown or overlooked, and as a comment it does not get enough visibility. \$\endgroup\$ – Dubu Feb 22 at 15:45
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    \$\begingroup\$ @Dubu I added it in the answer. \$\endgroup\$ – Michel Keijzers Feb 22 at 15:49
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    \$\begingroup\$ @MasonWheeler: you have to consider the maximum current rating of each item in the circuit, which may be much less than the rating of the circuit breaker feeding the outlet. If you plug a light duty extension rated at 5 Amp into the wall, then plug a 15 amp surge protector into that, you should not draw more than 5 amps total. \$\endgroup\$ – Peter Bennett Feb 22 at 16:47
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If you only plug low current devices into the extension cord there should be no problem. In any case, you need to keep the total current requirements of everthing plugged into the surge protector and extension cord below the rating of the surge protector. You also must consider the current requirement of anything else on the same circuit as the outlet the surge protector is pluggged into.

Manufacturers warn against plugging extensions or other power bars into a surge protector in case someone plugs a 15 amp heater into the last of three power bars.

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    \$\begingroup\$ I'd read somewhere that for surge protectors specifically, there are obscure reasons that it's not safe to plug one into another even with nothing else plugged into either - is there any truth to that? \$\endgroup\$ – Random832 Feb 22 at 14:38
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I'll add some information to what Michael and Peter wrote in their good answers.

You are right about lots of warnings being written everywhere on the subject. This is some sort of scarecrow tactic from the manufacturers (and rightly so, IMO).

Manufacturers must design their product and the included instructions around the (possibly dumbest) average consumer. Average people, sadly, have very little understanding of how electricity works: they confuse power, voltage and current all the time, let alone knowing what Ohm's law or Joule's law are!

So manufacturers don't take any chances. They tell you the simplest possible rule to use their products with safety: "One extension cord (adapter plug, or whatever) to one socket, no multiple adapter/extension things joined together". Stop. Simple and hopefully foolproof.

Of course, extension cords, plugs, sockets and similar "electrical connection things" follow Ohm's law and Joule's law. This latter is particularly important to calculate heating. If you know what you are doing and know the characteristics of your loads, you can mix and match almost anything by providing generous safety margins.

I have been running various multiple extension cords setups for ages, without the slightest problem. "Normal" people could watch my setups and cower in terror (if they are sane), or they could do the same and burn their house down, because they can't tell the electrical difference between a 4W power pack and a 1500W toaster! :-D

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Leaving whether it is safe or not (or widely practiced or not) to the others, I'll mention that in my U.S. jurisdiction the fire martial doesn't like it and made everyone in my company reconfigure practically every office to avoid this or face penalties.

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    \$\begingroup\$ There are also here (Netherlands) companies who do not allow a power strip connected to another power strip. The main reason is that people tend to either connect lots of computers to it (and in the past CRT monitors), or even worse, people connect coffee machines or other high current devices. In a company normally it is not checked on a regular basis if all power strips can handle the connected devices. \$\endgroup\$ – Michel Keijzers Feb 22 at 16:52